Infrared analyzers, especially those operating in the mid IR range with wavelengths of about 5-15.mu., are subject to changes in performance due to temperature effects. This is due to the fact that objects within the instrument housing give off radiation which will be sensed by the detector.
Analyzers in which the IR radiation is modulated by sequential insertion of filters or masks at timed intervals present a particular problem, in that the detector will sense some of the radiation reflected from and emitted by the filter and mask material per se.
Although this invention is described in application to a high speed rotary filter wheel type IR analyzer provided with reference and analytical optical filters which are interposed across the radiation beam at regular intervals, it can be embodied in filter type photometers broadly, including those using reciprocating filter flag or slide type modulation means. The radiation reflected from, and emitted by, the detector side of the modulating structure varies with temperature and can markedly affect the analyzer output signal, thereby reducing the measuring sensitivity. Mid IR analyzers are particularly susceptible to extraneous (stray) radiation because room temperature black body radiation peaks at a wavelength in the mid-IR range. The effect of such extraneous radiation on analysis is nullified by this invention by making the surface of the filter structure facing the detector as much alike as possible by using radiation non-conducting dummy ports corresponding to their counterpart analytical and reference filters and subtracting preselected detector output signals in a time pattern which exactly cancels out the extraneous radiation effects.
There are several different models of IR analyzers on the market designed to perform on-line analyses. Some of these employ a single beam of IR radiation which is modulated, as by individual radiation-selecting filters, to provide, in rapid sequence, an analytical wavelength for a preselected time interval and a reference wavelength for a different preselected time interval. This modulation can be achieved by mounting optical filters transmitting preselected radiation wavelengths on a support structure effecting interposition of the filters in preselected order at high speed across the radiation path.
It is preferred to utilize the detector analytical signal in ratio with the detector reference signal to give an output signal measuring the relatively fixed concentration of a constituent of interest in the radiation-transmitting sample. The reason for this is that, by using the ratio, the analyzer is rendered significantly less sensitive to factors which affect the signal at both of the reference and analytical wavelengths such as variations in source light level, dirt on the sample cell windows and the like.
The vexatious source of stray (extraneous) radiation sensed by the detector is that emitted by, or reflected from, the surface of the filter structure. This will not be a steady radiation source but will change, depending on what part of the filter structure is instantaneously viewed by the detector.
In the present state of the art several methods are now in use for temperature accommodation of mid-IR analyzers. One obvious way is to carefully control the temperatures of all sources of radiation which the detector might sense. This is an extremely costly way of eliminating the problem.
Another approach is to periodically have the analytical instrument recalibrated for baseline and span by the automatic insertion of the proper standard samples. This, too, is expensive and, besides, breaks the continuity of the analysis.
A principal object of this invention is to provide a method of temperature compensation which is inexpensive and does not reduce the sensitivity of the analyzer.